Requiem Threats: agonistic displays

in the Grey Reef and other sharks

The deep sense of peace I usually feel while diving was not
there. November 1982: I am 27 m (90 feet) below the surface on a ledge at the
northern tip of Osprey Reef – some 230 km (140 miles) off the Queensland coast.
And I am about to ‘attack’ one of the fastest and best-armed creatures that
swim.

Feeling the need for reassurance, I concentrate on the heft of
my DPV (Diver Propulsion Vehicle); it doesn’t seem nearly as solid as it did on
deck. The time is 11:01. Any minute now, the big female Silvertip Shark (Carcharhinus
albimarginatus) I know as “Notchfin” will be cruising by this spot, heading
south at a leisurely 1.5 knots – just as I have seen her do a half-dozen times
before. Although it is obscenely hot in the glaring sun topside, down here it is
surprisingly chill; I begin to shiver.

11:03 – right on schedule – a flicker of movement in the
diffuse half-light betrays her approach. With terrifying suddenness, the
spectral blur resolves into crisp focus, a deep V-shaped cut on the forward edge
of the first dorsal confirming her identity. Notchfin is about 1.7 m long,
probably not yet mature; except for her injured fin, her lines are clean with no
signs of mating scars. She moves with the liquid grace of a creature that is
perfectly comfortable with its role in the universe. Her presence is nothing
short of electric. My awareness of the viscous cold evaporates and for a moment
we are suspended in an incredible blue nothingness. She is a marvel: sleek and
solid and – I remind myself – potentially lethal.

This realization snaps the spell. Less than 15 m (50 feet) of
water separate us; my breathing seems awfully loud. With exaggerated precision,
I raise the DPV and point it toward the oncoming shark. How will this wild
animal react to my charging toward it? Swallowing dryly, I tighten my grip on
the DPV and flick the switch to “ON” …

Conventional wisdom holds that sharks are “unpredictable”. However, a certain
type of Indo-Pacific shark has long been known to consistently signal its
readiness to attack by a complex rolling display. When persistently approached
by divers, the Grey Reef Shark (Carcharhinus amblyrhynchos) raises its
snout, depresses its pectoral fins, arches its back, and flexes and holds its
tail sideways. This posture is often accompanied by laterally exaggerated
swimming in a yawing “figure-eight” pattern. This display increases in intensity
when the animal’s escape route is restricted. If a diver continues to approach,
this shark launches a lightning-fast, slashing attack followed by a rapid
retreat. From the results of their classic study in Micronesia, researchers
Richard H. Johnson and Donald R. Nelson concluded that the display of the Grey
Reef Shark “appeared ritualized in nature and is likely to be of value in normal
social encounters”. Every time either researcher approached a Grey Reef Shark,
the same display was elicited. Here we have a case where at least one species of
shark, when presented with a specific (admittedly artificial) situation, reacts
predictably.

The distinctive threat display of the Grey Reef Shark (Carcharhinus
amblyrhinchos) indicates that the animal is on the verge of either attacking
or fleeing; similar displays have been elicited by the author in several other
shark species. If you observe a shark lower its pectoral fins and change its
swimming style, move away from the animal while maintaining eye contact with it.
Often, this action causes a displaying shark to seize the opportunity to make
good its escape.

As a diving naturalist particularly interested in the behavior of sharks, I
just had to see this display for myself. Since we had the very same species of
shark off the Queensland coast, I decided to attempt to provoke one of ‘our’
Grey Reef Sharks into displaying, following the methods of Johnson and Nelson.
Much to my disappointment, our Australian Grey Reefs didn’t display – they
merely dropped their pectoral fins and accelerated away for a distance of about
12 m (40 feet), then proceeded on their aristocratic way like a James Bond
martini: shaken, not stirred.

Undaunted, I decided to try the same technique on another, closely related
species of shark. I knew when and where to intercept a certain individual
Silvertip Shark, identifiable by her damaged first dorsal fin. But I made one
modification: the addition of a DPV. Since no one could have told me what to
expect, I wanted something solid between me and the shark. Having stationed
myself at a strategic site along Notchfin’s route, I planned to wait motionless
until she approached to within about 15 m (50 feet). I decided to use the DPV
only to pursue the test subject and to switch it “OFF” as soon as the animal
began to display.

Notchfin didn’t seem to notice my motorized approach until I
was about four m (13 feet) away. Her immediate reaction was disappointingly
familiar: she dropped her pectoral fins and accelerated away until she was about
20 m (60 feet) distant, then assumed her former cruising speed. I changed
direction and charged after her again. When the approach distance was 3.5 m
(11.5 feet) – about two body lengths – Notchfin dropped her pectoral fins,
darted some five or sic body lengths away … and then came charging toward me
with her pectorals strongly depressed. As my mind snapped violently into
Dreamtime, I remember thinking – quite calmly – “This is it, mate: you’re about
to snuff it.”

Then a very curious thing happened. When she got within about
two body lengths, Notchfin turned broadside to me and greatly slowed her forward
movement. The posterior two-thirds of her body were lowered and held tense (in
what I interpreted as a threat posture, akin to an arch-backed Halloween cat)
and began to “shiver”. Her form was reduced to a blue-grey blur; the white
apices of pectoral, first dorsal, and upper caudal fins (which inspired the
vernacular name for her species) fairly glowed in the darkling light. She
continued to display like this until her mid-body had more-or-less passed by me
(Perhaps when I was within her “blind spot”?). She then straightened her body
and shot away with her pectoral fins still strongly depressed …

When my adrenaline levels subsided enough to afford something resembling
rational thought, several points about the interaction struck me as particularly
interesting. For openers, there was Notchfin’s apparent reluctance to confront
or challenge me. Of course, her repertoire of instinctive and learned responses
probably included precious little about “What to do if attacked by a crazy
Banana-bender driving a DPV”, so my charge must have had a certain “shock
value”. But what I found most intriguing was that Notchfin showed little or no
signs of being threatened until I approached within two body lengths’ distance
and did not bring into play the “shiver” element of the display until she
approached within two body lengths. It seemed as if the shark was punctuating
the fact that a clearly defined (in her mind, at least) boundary had been
violated.

Over a 10-year period, I had been able to elicit this display in 8 out of 28
trials, involving at least five individual Silvertips. This strongly suggests
that Notchfin’s display was typical of Silvertip Sharks when threatened in this
manner. The threat display of the Silvertip Shark shares certain features with
that of the Grey Reef Sharks studied by Johnson and Nelson, yet includes some
totally different elements. Pectoral fin depression, flank displaying, and an
overall increase in muscular tension are common to the displays of both species,
but vertical depression of the posterior portion of the body and the “shiver”
are unique to the Silvertip threat displays. I have since expanded my study of
shark threat displays to 25 species of sharks, representing a total of eight
families. Each shark species has a display with a unique combination of
features, often including at least one element not exhibited by other species.
However, for all their variety, the threat displays of all sharks studied to
date feature: sustained pectoral fin depression, exaggerated swimming movements,
an overall increase in muscular tension, and are terminated by rapid withdrawal.

Animals have evolved many complex and subtle ways to communicate with one
another, including specific scents, patterns of sounds, colors and markings.
Often, these features are accentuated by combining them with behavioral elements
to more clearly communicate such things as species identity, readiness to play,
fight, or mate. A “display” may be defined as a stereotyped behavior, or series
of behaviors, modified by evolution to communicate. Displays are often derived
from non-communicative behaviors whose elements are exaggerated, slowed, and
otherwise modified (“ritualized”) until they take on a new meaning. Most
displays are species-specific and concerned with reproductive or agonistic
behaviors. “Agonism” is a convenient blanket term for any behavior arising in
competitive contexts. The term embraces a broad spectrum of behaviors, including
space and distance regulation, displacements, defensive threats, escape,
submissive gestures, and self-assertive behavior.

What are displaying sharks trying to communicate? We may never know for sure,
but a few general principles can be deduced. Since most – if not all – displays
known to date occur in approach-withdrawal conflict situations, they may
properly be termed agonistic in nature. Despite Tennyson’s colorful description
of Nature “red in tooth and claw”, actual physical combat is rare among animals.
If the most basic and powerful drives among living things are to survive and
reproduce, it doesn’t make sense for an individual to risk a tussle which could
result in injury and thus interfere with its ability to feed and/or potential
for representing its genes in future generations. This is why so many animals
posture and display toward one another in competitive situations: the idea is to
bluff one’s opponent into backing down without having to engage in actual
fighting.

Johnson and Nelson stated that the stereotyped displays they observed in Grey
Reef Sharks tended to occur in situations unfamiliar to the shark. The
exaggerated, ritualistic nature of these displays marks them as defensive rather
than competitive or aggressive. Therefore, a rapidly approaching diver is
probably perceived by a shark as a potential threat. Behaviorist George Barlow
has suggested that the agonistic displays of sharks may represent a kind of
motivational conflict, or “indecision”, between fleeing (escape motivation) and
attacking (defense motivation). In my experience, sharks are basically “path of
least resistance” types: given the opportunity to flee rather than fight, most
sharks will simply swim away.

The nearly horizontal angle of the pectoral fins in
this Grey Reef Shark (Carcharhinus amblyrhynchos) indicate that it is
relatively 'at ease' with the photographer's presence.

Scuba has proven itself to be a powerful tool for investigating the
behavior of sharks in the context of their natural environment. It is
becoming increasingly clear that sharks are far from the unpredictable,
socially unsophisticated creatures we had once mistaken them for. We now
know that sharks typically adhere to well defined “home ranges” and that
their daily and seasonal movements are highly regular and predictable.
Although they have a strong sense of home turf, individual sharks will
tolerate other sharks – including members of the same species – swimming,
and even feeding, within their home range. Thus, sharks cannot be said to be
“territorial” – that is, defending a specific geographical area for their
exclusive use.

My own research has also revealed that sharks, like people, have a sense of
“personal space” (sometimes termed idiosphere*) with
well-defined boundaries – a kind of moving territory, which they definitely will
defend. This personal space is roughly spherical in shape and its radius is
directly proportional to the size of the animal. My work on reef-dwelling whaler
sharks (genus Carcharhinus) has borne out a pattern of a critical
approach distance of about two body lengths.

This length proportional defended volume strongly supports the idea that
sharks have a good awareness of their own body size. While observing sharks
under baited conditions, I and other workers, have noticed a definite
size-related “pecking order”. In our own species, the personal space defended by
individuals is also related to social rank, with higher ranking humans having
larger personal spaces. As social vertebrates, we may have more in common with
sharks than we realize.

* often misspelled “kinosphere”
(which refers to movement space, as used in choreography). Use of this term is
rare, highly idiosyncratic, and seems to be limited to human psychology. I
recently proposed the general term “idiosphere” (from the Greek idios = own) for
the volume surrounding an individual which it will defend if broached without
permission or appropriate signaling. As such, this term extends the well-defined
concept of personal space to non-human animals. Time will tell whether the term
is a useful one and becomes widely used.